Marine vehicles are generally unanchored or untethered to any solid ground. The nature of being out in open water with no references points can make navigation difficult. In fact, it can be so difficult that even maintaining a fixed position on the earth is challenging. Despite the challenges, in some cases maintaining such a fixed position over long periods of times is essential. For example, when a drilling vessel extends a drilling riser through the water to the subsurface to drill for oil, the position of the drilling riser is critical to making a correct incision into the underground well. Further, once the drilling riser is in place, the vessel must maintain its position in order to prevent the drilling riser from disconnecting from the subsurface. Maintaining this position is referred to as “station keeping.”
Conventionally, station keeping is performed by way of a Dynamic Positioning (DP) control system on marine vessels. The DP control system has a basic mathematical requirement to establish and calculate various key parameters. These mathematical models incorporate equations of motion, which are used to characterize the pose and dynamics of the vessel. The dynamics of a marine vessel include six degrees of freedom (DOFs) for motion. These DOFs are surge, sway, heave, roll, pitch and yaw. Three parameters in particular are given more weight, and those three are surge, sway and yaw. Thus, the DP system's primary role is to maintain the surge, sway, and yaw set point by way of a control system that sends commands to the vessel's thruster drives. A DP drive-off (DO) occurs when a dynamically-positioned vessel erroneously has a position excursion from the desired set point. This can happen for various reasons, some of which are related to erroneous spatial observations provided by different sensors, such as GNSS, draught, and/or acoustics. If the mathematical model consuming these observations arrives at a state with high confidence in an erroneous measurement, the DP system's force and moment demands to the thrusters may be in error as well.
A DO poses a challenge to a vessel operator because even an experienced operator may not be able to identify if the control system is reacting to the environment correctly or if there is an error that is resulting in a DO. If a deficiency in the observables or model is identified the operator must isolate the source of the error. This process must be done accurately and in a relatively short amount of time.